The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 7,085,623; 7,082,359; 7,080,303; 6,966,048; 6,948,153; 6,259,890; 6,195,528; 5,915,973; 5,827,070; 5,565,316 and 5,211,564; and
U.S. patent application Publication Nos. 2006/0107254; 2006/0088811; 2004/0229199; 2003/0203342; 2003/0196170; 2003/0182602; 2003/0138765 and 2003/0129573.